Changes in epicardial and endocardial direct current (DC) electrograms and ventricular arrhythmias observed during 60 min of left anterior descending coronary artery (LAD) occlusion in nine anesthetized pigs were compared with those observed in eight other pigs after embolization of the LAD with latex, a procedure able to block collateral blood flow to the ischemic area. After LAD occlusion 1) T-Q segment depression and S-T segment elevation showed a faster rate of development, and monophasic potentials occurred earlier in the endocardium than in the epicardium; 2) T-Q segment depression was greater at the center than at the periphery of the ischemic area during the first 20-30 min, and later it become greater at the periphery than at the center; and 3) a period of transient recovery in the local activation beginning after 8-15 min and lasting for 10-20 min occurred in all cases, commonly associated with T-wave alternans. LAD embolization with latex induced greater T-Q and S-T segment changes in the epicardium than in the endocardium, increased the incidence of ventricular fibrillation, and failed to impede the appearance of the period of transient electrical recovery. Thus, within an acutely ischemic area in the in situ pig heart, there are inhomogeneities in the magnitude and time course of the electrical changes, which are not prevented by procedures able to block collateral blood flow to the ischemic area. Ventricular fibrillation was also not prevented by such a procedure.